1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to multi-cell cooperation in a wireless communication system.
2. Discussion of the Related Art
A multiple-input multiple-output (MIMO) system is a system which uses multiple transmit antennas and multiple receive antennas to improve transmission/reception efficiency. In theory, the MIMO system has maximum channel capacity when using dirty paper coding in which data of other users is removed in advance by a base station (BS) when the data acts as interference to each user, and thus interference from other users is reduced. However, the dirty paper coding has a problem in that it is difficult to be implemented in an actual system since not only a transmitter requires much channel information but also computational complexity is high. Various schemes for concurrently allocating spatial resources to a plurality of users have recently been proposed so that the principle of the dirty paper coding can be implemented in practice.
An example of selecting a precoding matrix is proposed in per user unitary and rate control (PU2RC) disclosed in a contribution document R1-060335 “Downlink MIMO for EUTRA” provided by Samsung Electronics Ltd. In this example, each user selects a precoding vector capable of maximizing a channel data rate of each user from a plurality of precoding matrices, and feeds back an index of the selected precoding vector and a signal to interference plus noise ratio (SINR) to a BS. On the basis of information fed back from each user, the BS selects a precoding matrix and a user.
Research on a multi-cell cooperative system capable of obtaining a diversity gain by using cooperative communication of a plurality of BSs has actively been conducted in recent years. The multi-cell cooperative system has been introduced to provide cell coverage extension, throughput enhancement, performance enhancement in a cell edge region, etc.
In a system in which a feedback is limited, a multi-cell cooperative scheme can be divided according to a level of information sharing between BSs. That is, the multi-cell cooperative scheme can be divided into: a first scheme in which channel information and transmission data information are not shared between BSs; a second scheme in which only channel information is shared and data information is not shared between BSs; a third scheme in which only data information is shared and channel information is not shared between BSs; and a fourth scheme in which channel information and data information are both shared between BSs. It is expected that the fourth scheme has best performance in theory, but if all BSs participating in the multi-cell cooperation share data and channel information of all users, a load of a backhaul network increases and thus a problem may arise in actual system implementation. A precoding method using a scheme of sharing both channel information and data information between BSs is disclosed in a contribution document R1-084114 “Per-cell precoding methods for downlink joint processing CoMP” proposed by Electronics and Communications Research Institute (ETRI).
When data information of users belonging to all BSs participating in cooperation is shared between BSs in the multi-cell cooperative system, a load of a backhaul network and a buffer size of a BS are increased. This may act as a significant burden in the designing of a wireless communication system that requires high-speed data transmission.